The present invention relates to a control device for a converter used in a DC transmission system.
Generally a DC transmission system comprises main transformers, two converters, control devices for the converters and DC transmission lines. Either converter is comprised of thyristor valves. One of them is a rectifier for converting AC power to DC power, and the other is an inverter for converting DC power to AC power. The control devices are used to control the phase angle of gate pulses to be supplied to the thyristor valves of either converter to thereby adjust the DC transmission power. Usually, constant current control is performed on the rectifier and constant margin-angle control is performed on the inverter. Nevertheless, an automatic current regulator and an automatic margin-angle regulator are provided for either converter. This is because the direction of the tidal current in the DC transmission system may be inverted. Output signals from both regulators flowing through a low value gate are converted to firing pulses. The firing pulses are supplied to the gates of the thyristor valves. The margin-angle setting value of the automatic margin-angle regulator is usually about 19.degree. in order to avoid a commutation failure in the inverter operation mode. The current reference of the automatic current regulator for the inverter is made smaller than that of the rectifier by the amount of a current margin. Hence, in the control device on the rectifier side the output from the automatic current regulator precedes the output from the automatic margin-angle regulator and the opposite is true of the control device on the inverter side.
In the DC transmission system described above, DC voltage Ed is given as: ##EQU1## where, E is secondary winding voltage of the main transformers, .gamma. is margin-angle, x is commutation reactance (P.U.), i is DC current (P.U.), and e is commutation voltage (P.U.).
Commutation reactance x is about 20% based on the main transformer capability. This value is determined by the thyristor overcurrent capability in order to cope with a failure of the converters. As clearly understood from equation (1), if margin-angle is fixed, DC voltage Ed will change approximately 10% when DC current i changes from 0 to 1 P.U.
In order to prevent DC voltage Ed from changing, an automatic voltage regulator may be employed, in addition to the automatic current regulator and the automatic margin-angle regulator. The third regulator performs constant voltage control on the inverter. If the three regulators are negative feedback control circuits, an undesirable phenomenon may take place. When the automatic voltage regulator is operating in the inverter operation mode, the automatic margin-angle regulator is saturated. Its margin-angle .gamma. will temporarily become smaller than 19.degree. when the AC voltage fluctuates. This is because the automatic margin-angle regulator cannot operate rapidly because it is saturated. This causes the reduction of margin-angle .gamma. which results in a commutation failure.